With progress of multimedia networks, the demand for communication traffic is rapidly increased. Optical transmission apparatuses for relaying wavelength-division multiplexed (WDM) signal light, which is obtained by combining a plurality of signal lights of different wavelengths, using an optical amplifier that includes a rare-earth-doped optical fiber as an amplification medium play a major role in economizing on communication systems in a multimedia society.
FIG. 1 is a block diagram illustrating an example of an optical amplifier. For example, as illustrated in FIG. 1, an optical amplifier for amplifying WDM signal light includes a first erbium-doped fiber (EDF) 111, a loss medium 112 (for example, a gain equalizer or a dispersion compensator), and a second EDF 113 that are optically coupled in series on a signal path between an input port IN and an output port OUT. With the optical amplifier in which the loss medium 112 is disposed between the EDFs 111 and 113, both the degradation in a noise figure (NF) that occurs in an optical amplifier in which a loss medium is disposed at a stage prior to a single EDF and the increase in required excitation light power that occurs in an optical amplifier in which a loss medium is disposed at a stage subsequent to a single EDF can be solved at the same time. Accordingly, the optical amplifier illustrated in FIG. 1 has a configuration efficient for achievement of performance required for optical amplifiers.
Referring to FIG. 1, excitation light Lp is transmitted from an excitation light source 121 to one end of the first EDF 111 via a multiplexer 122. A part of the excitation light Lp that has not been absorbed by the first EDF 111 is leaked out from the other end of the first EDF 111. A path used to transmit the leaked excitation light Lp to the second EDF 113 without attenuating the leaked excitation light Lp in the loss medium 112 is formed. More specifically, a multiplexer 123 disposed between the first EDF 111 and the loss medium 112 transmits the part of the excitation light Lp leaked out from the first EDF 111 to an alternative path 124. The part of the excitation light Lp transmitted through the alternative path 124 is supplied to the second EDF 113 via a multiplexer 125 disposed between the loss medium 112 and the second EDF 113. Thus, by forming a path for excitation light that bypasses the loss medium 112 disposed between the first EDF 111 and the second EDF 113, excitation can be performed on the EDFs 111 and 113 with the excitation light source 121. As a result, the cost reduction of an optical amplifier can be achieved. Such an optical amplifier including an alternative path for excitation light is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2000-77757 (see, for example, FIG. 5.)